Formula: NiS Hexagonal
Description: Millerite is a low temperature hydrothermal mineral found in vugs in carbonate rocks and as an alteration product of other nickel minerals. Millerite typically forms long brassy fibers making very aesthetic specimens. It becomes green or dulled as it alters to other nickel minerals such as violarite, bravoite and honessite.
GRANT COUNTY: Millerite is found associated with enargite, sphalerite, calcite, chalcopyrite, marcasite and pyrite at the Piquette #2 mine, SW NE Sec. 36 T.3N R.3W near Tennyson (Heyl, 1964).
KENOSHA COUNTY: Millerite occurs as a heavy mineral found in cores from a subsurface, diamond-bearing lamprophyre diatreme, discovered within the outskirts of Kenosha (Carlson and Adams, 1997.).
IOWA COUNTY: Millerite has been observed in several mines in the Linden district where it is found in “radical acicular clusters up to an inch in diameter” in vugs, adhering to sulfides, in calcite, or along fractures in the dolostone country rock. Associated mineral include octahedral galena xls., chalcopyrite xls., marcasite, and sphalerite. The millerite is frequently associated with violarite and nickeliferous pyrite (bravoite). It oxidizes to bonessite (Heyl, Milton and Axelrod, 1959). Some localities are: the Glanville Mine (SW SW Sec. 8 T.5N R.2E.) the Mason Mine, (SE SW Sec. 8 T.5N R.2E.) and the Gilman Mine (SW NE Sec. 16 T.5N R.2E).
LAFAYETTE COUNTY: Millerite is reported as a sparse component of the zinc-lead ores in of the Rewey and Mifflin quadrangles (Taylor, 1964).
— Millerite is found as crystals pseudomorphously replaced by violarite at the Farrey mine along Wisconsin Highway 11, NE NW Sec. 14 T.1N R.1E, at Leadmine (Heyl, Milton and Alexrod, 1959).
— Millerite is found at the Dodgeville #2 mine, Dodgeville N 1/2 NW Sec. 34 T.4N R.3E (Heyl, Milton and Alexrod, 1959).
— Millerite occurs at the New Cottington Mine, SE SE NW Sec. 13 T.1N R.1E (Heyl et al., 1959).
— Millerite is found at the James Mine, Sec. 9 T.1N R.2E near Shullsburg (Heyl, Milton and Axelrod, 1959b).
MILWAUKEE COUNTY: Some of the most beautiful mineral specimens found in Wisconsin are the millerite clusters from the Milwaukee River channel at Estabrook Park, Sec. 5 T.7N R.22E. The deposits are described by Bagrowski (1940). The millerite forms individual stout hairs and radiating groups with individual crystals up to 3 inchs long. Associated minerals are complex calcite crystals, pyrite crystals, iridescent marcasite crystals, sphalerite crystals, and zaratite. The millerite is found in two cavernous layers within the Milwaukee formation. Asphaltum is also found in these layers, but not in the same vugs as the millerite. Labels on specimens from this locality refer to it by a variety of names including the Capitol Drive, Washington street bridge, North Milwaukee, Lincoln Park and Berthelet.
— Millerite was found at the “old Milwaukee Cement quarry” with calcite. The millerite occurs in fossils and other cavities as needles, “close sheaves, open radiations… or as wool masses whose hairs are interwoven” (Rawson, 1931).
WOOD COUNTY: Millerite is present as rare tiny fibers in calcite in veins cutting the granite exposed in the Haske quarry in Sec. 25 T.24N. R.2E (Tom Buchholz, personal communication).